The GIA grading terminology uses a combination of fancy grades and color descriptions to identify a colored diamond’s characteristic color. A fancy grade represents the combined effect of tone and saturation on the color of a diamond. These grades correspond to regions of tone and saturation in color space and vary by hue, since different colors reach their highest saturation at different levels. The color descriptions accompanying a fancy grade are determined by the hue, and by the tone and saturation of the hue. In each instance, the fancy grades and color descriptions represent a range of color appearances.
Saturation refers to the dominance of hue in the color, and Intensity of it. Saturation is the measure of how strong and intense the primary color of the diamond actually is, such as light, deep, intense or vivid. A fully saturated color is the truest version of that color. Primary colors (red, yellow, and blue) are "true", so they are also fully saturated.
There are six categories in the GIA scale that equate to color saturation. For more information please visit GIA